Digital wireless communications are being widely used throughout the world particularly with the latest development of the Orthogonal Frequency Division Multiplex (OFDM systems) and the latest evolution, namely the so-called Long Term Evolution (LTE), DVB-H, WiFi 802.11 and WiMax 802.16 systems. It is well-known by one skilled in the art that OFDM communication systems performances are severely impaired by the rising of inter-carrier interference (ICI) effect in the presence of time-varying propagation channels. Such circumstances occur in mobile cellular OFDM communication systems envisioned in the above mentioned standards.
It is well-known that classical OFDM low-complexity detection techniques are severely impaired when the user equipment (UE) moves at high velocity. These conditions result in fast time-varying propagation channels and a high Doppler spread, which in turn yields to significant ICI. The so increased ICI prevents classical OFDM receiver schemes from reliably detecting the desired signal. Hence more advanced receiver equalization techniques are required to mitigate the effect of the ICI.
It is also well-known from the literature that a solution to the above ICI mitigation-equalization optimization problem is readily found when the receiver includes the complexity of a full matrix inversion. In existing telecommunication systems, such a full channel matrix inversion operation cannot be supported by a practical mobile receiver.
Hence several approaches have been introduced to reduce the complexity of the above optimization problem. To this end the use of time-domain windowing of the OFDM symbol has been shown to limit the span of inter-carrier interference and allow for frequency-domain iterative detection exploiting the voluntarily-generated banded nature of the new channel matrix.
FIG. 1 illustrates the general principle of a prior art technique using a time domain window. One sees that, in the transmitter part of the OFDM system, the frequency domain signal s is being converted in the time domain by means of a inverse Fourier Transform block 11 before being serialized by a parallel to serial block 12 and transmitted through a propagation channel represented by block H. In the receiver, a time domain windowing is applied on the signal received from the antenna before the serial to parallel conversion by a block 15 and the Fourier transform by block 16. Time domain windowing is based on the multiplication of the samples of one Block by coefficients embodying a window. Such operation has the effect of controlling the inter-carrier interference (ICI), i.e. the interference among different subcarriers, and causes the limitation of the span of the matrix FWHFH between the transmitted sequence s and the received signal r.r=FWHFHs
The result is that equalization is facilitated via a span limited matrix.
However this known technique has a serious drawback in that the time domain window results in a loss of energy of the received signal due to the suppression of some samples of the received signal.
FIG. 2 shows an equivalent model of a prior art technique which, consists of a time domain windowing block 21, a Fast Fourier transform block 22, and then is followed by a Decision Feedback Equalizer embodied by a forward filter 23, adder-subtractor 24 and a backward filter 25 arranged in a feedback loop. The loss of information resulting from the time domain windowing has the result of jeopardizing the convergence of the equalizing process.
Such a technique becomes unsatisfactory when the Doppler effects are increased because of the loss of information resulting from the time domain windowing being performed.
Such are the technical problems addressed by the present invention.